Using His bundle electrogram recordings and atrial pacing techniques the following studies will be conducted: 1. The effect of pharmacologic agents on the electrophysiological properties of the A-V nodal and His-Purkinje conduction system at sinus and various paced atrial cycle lengths. 2. The determinants of reentry within the region of the sinus node and A-V node. 3. The determinants of Gaps in A-V conduction phenomena (Types I and II). 4. Mechanisms of arrhythmias in anomalous A-V conduction.